Apple owns up to odd—but serious—Snow Leopard data-loss bug

A serious data-loss bug is affecting a small percentage of users who have …

Snow Leopard has, in our experience, been a mostly problem-free upgrade. However, an unusual and potentially serious bug has been identified related to guest accounts and Snow Leopard upgrades. Unfortunately, using a guest account has the potential to wipe your own user account clean of all data, though Apple has acknowledged that it is aware of the problem and is working on a solution.

The problem doesn't affect every Snow Leopard user—it seems to be linked to changes in the way guest accounts are handled between Leopard and Snow Leopard. To trigger the bug, a machine running Leopard with the guest account feature enabled must be updated to Snow Leopard. The next time the guest account is accessed, it appears to wipe the user's main home directory clean, as if it were also a guest account.

"We are aware of the issue, which occurs only in extremely rare cases, and we are working on a fix," an Apple spokesperson said in a statement Monday.

MacFixIt first noted the problem in early September, as users who experienced the problem began posting about the issue in Apple's support forums. Later, MacFixit identified a way to avoid the problem: disable the guest account before updating, and re-enable it after updating. Or better yet, if you don't really need guest accounts, just leave them turned off.

Unfortunately, unless you have a recent Time Machine or other backup, your data may be gone for good—unless you're willing to pay for an expensive data recovery process. But then again, Ars readers are they sort that have regular backups, right?

Apple is currently seeding a beta of Mac OS X 10.6.2 to developers for testing, though there is no information that it addresses this bug specifically. Given the potential for massive data loss, though, we expect that fixing the problem is a priority for Mac OS X engineers.